In recent years, monitoring and treatment of patients at home becomes more and more important in view of costs as well as convenience. For these patient monitoring systems, sensors are required that are easy to use and do not degrade the compliance of the patients. For instance, in order to treat patients with chronic respiratory failure at home, future non-invasive ventilation systems may incorporate a module for non-invasive measurement of blood gas concentrations to check the effectiveness of ventilation. For non-invasive CO2 blood gas monitoring, transcutaneous sensors (measurement through the skin) seem to be the best solution.
In general, transcutaneous sensors consist of a membrane, which is permeable for small neutral molecules in the gas or vapour phase (like CO2, O2, N2, and H2O), but not for ions nor large molecules, and of a sensor fluid, into which the gas molecules diffuse. In the example of CO2 sensors, the fact is used that, when CO2 dissolves in water, part of it will form carbonic acid (H2CO3), which in another equilibrium reaction with H2O forms H3O+ and HCO3−. By measuring the amount of these ions, the CO2 pressure on the other side of the membrane, which is in equilibrium with the sensor fluid, can be derived.
However, current transcutaneous systems are optimized for prolonged use in the hospital and need placement of the sensor on the skin by trained staff. A further disadvantage of these sensors is that they need replacing of the membrane and of the sensor fluid approximately every two weeks (so-called re-membraning). This is required on the one hand, since the membrane gets dirty on the outside, e.g. by fatty substances from the skin, which block the membrane pores through which the gas molecules to be detected should flow. On the other hand, conventional transcutaneous sensors suffer from sensor fluid loss, such as fluid evaporation through the membrane. Furthermore, ions leach out from the sensor housing, glue or other sensor parts into the sensor fluid, thus polluting the sensor fluid. These unwanted “polluting” ions in the sensor fluid shift the equilibria of the above-mentioned chemical reactions and distort the measurement results. Hence, conventional fluid-based sensors for fluid-soluble gas molecules are subject to drift during the measurements and need calibration before use. Therefore, these prior art sensors are rather inconvenient to operate and require expertise in application and maintenance, thus inappropriate for use at home or by the patient himself.
U.S. Pat. No. 4,228,400 describes a conductometric gas analysis cell comprised of a porous Teflon membrane supporting interdigitated electrodes, which separates a thin layer of deionized water from a gas phase to be analyzed.